1. Field of the Invention
This invention relates to the field of circuit design and, more particularly, to a system and method for securing data used to configure a programmable logic device (PLD).
2. Description of the Related Art
A PLD is a general purpose programmable device that is customizable by an end user to realize a desired user-specific device. One example of a PLD is a field programmable gate array (FPGA). The basic FPGA architecture involves an array of configurable logic blocks (CLBs) embedded in a configurable interconnect structure and surrounded by configurable I/O blocks (IOBs). In U.S. Pat. No. 6,496,971, Lesea and Trimberger have described an FPGA configuration structure where configuration is carried out by a microcontroller, for example a microprocessor. Configuration is the process of loading configuration data into the configuration memory cells which control the programmable interconnect structure, the IOBs, and the CLBs. The microcontroller may be used to read configuration data and write it to configuration memory cells. If the configuration data is encrypted, the microcontroller must also decrypt the data. A significant fraction of the configuration logic can be the bitstream decryptor. The decryptor would be a prime target for implementation in the microcontroller because it is large, is used rarely, and there are several different decryption algorithms that one might use.
In order to reduce the amount of dedicated read-only memory on the chip and to permit updates to the configuration process, the microcontroller configuration boot program can be loaded from an external source when the FPGA configures. A decryption key may be stored in a volatile or non-volatile register. A common method of connecting this register, whether volatile or non-volatile, is by the well-known method of memory mapping. Using memory-mapping, the system includes a decoder and enabling logic that permits the register to be read and written by the microcontroller as if it was ordinary memory. The simple method of storing and using decryption keys as a memory-mapped register connected to the microcontroller is un-secure and can lead to a pirate stealing the key data or the decrypted design by providing a “Trojan-horse” configuration boot program that writes the contents of the key register to a block random access memory (BRAM) or to chip IOs. What is needed is a structure and method that permits a microcontroller to store and use decryption keys in a secure manner.